The present invention relates to an objective lens, an optical pickup device and a recording and reproducing apparatus, all for conducting at least one of recording and reproducing of information for an optical information recording medium.
Researches and development have been advanced in recent years for a novel high density optical disk system employing a light source of a violet semiconductor laser emitting light with a wavelength of about 400 nm and an objective lens whose numerical aperture (NA) has been enhanced up to 0.85. As an example, in the case of an optical disk having a numerical aperture of 0.85 and light source wavelength of 405 nm (hereinafter called “high density DVD), information of 20–30 GB per one surface can be recorded on an optical disk having a diameter of 12 cm which is identical to that of DVD (numerical aperture 0.6, wavelength of light source 650 nm and storage capacity 4.7 GB).
When a violet semiconductor laser emitting light of a short wavelength of about 400 nm is used as a light source, in this case, chromatic aberration caused on an objective lens is a problem. In an optical pickup device, chromatic aberration of an objective lens is considered not to be a problem because a laser beam emitted from a semiconductor laser is generally of a single wavelength (single mode). Actually, however, there is caused mode hopping in which a central wavelength is changed instantaneously by several nanometers by temperature changes and output changes. Since the mode hopping is a wavelength change occurring instantaneously which cannot be followed by a focusing mechanism of an objective lens, defocusing component corresponding to an amount of movement of an image forming position is added if chromatic aberration of the objective lens is not corrected, thus, light-converging power of the objective lens is deteriorated.
Since dispersion of general lens material used for an objective lens is not so great in 600 nm–800 nm representing a wavelength area for infra-red semiconductor laser and red semiconductor laser, deterioration of light-converging power of an objective lens caused by mode hopping was not a problem for CD and DVD.
However, dispersion of lens material is remarkably great in the vicinity of 400 nm representing a wavelength area of a violet semiconductor laser, therefore, an image forming position of an objective lens is shifted greatly even by a wavelength change I a mere several nanometers. In high density DVD, therefore, when mode hopping takes place on a light source of a violet semiconductor laser, light-converging power of the objective lens is deteriorated greatly, resulting in a fear that recording and reproducing are not carried out stably.
Incidentally, in the case of an optical pickup device, plastic lenses are commonly used as an objective lens because the plastic lens is advantageous for mass production. However, it has been known that the refractive index of the plastic lens is greater than that of a glass lens by about two figures.
When ambient temperature for the objective lens made by plastic material rises, and the refractive index of the objective lens is changed, spherical aberration of the objective lens is deteriorated. Since an amount of deterioration of the spherical aberration caused by the change of the refractive index is proportional to the fourth power of the numerical aperture of the objective lens, when an objective lens with numerical aperture of 0.85 used for high density DVD is made to be a plastic lens, a range of usable temperatures is narrowed remarkably to be a problem in practical use.
Hereupon, deterioration of spherical aberration caused by chromatic aberration of an objective lens and by refractive index changes is generated in proportion to a focal length of the objective lens. Therefore, if the focal length of the objective lens is made short for the problems mentioned above, chromatic aberration of the objective lens and deterioration of spherical aberration caused by refractive index changes can be controlled to be small, even when a violet semiconductor laser having a short wavelength and a plastic objective lens having a high numerical aperture are used.
Incidentally, the preferable one for realizing an objective lens having a numerical aperture of 0.85 is a two-group objective lens wherein the refracting power of an optical surface for a ray of light is divided into four surfaces, sensitivity for manufacturing error of each lens is small and manufacturing is easy. A lens having a high numerical aperture and structured by 2 groups is disclosed by Tokukaihei 10-123410, Tokukaihei 11-190818 and Tokukai 2000-20640. Further, an example to make a focal length shorter is also disclosed. However, there are the following problems in prior art.
As stated above, the smaller a focal length of an objective lens is, the smaller the deterioration of spherical aberration caused by refractive index changes is controlled, however, if the focal length is made small in a two-group objective lens, there are feared the following problems;
(1) a distance between the final surface of an objective lens and an optical disk (so-called the working distance) becomes too small to increase possibility of collision between an optical disk and an objective lens, and
(2) an outside diameter of each lens constituting a two-group objective lens, in particular, of a lens arranged to be closer to an optical disk, becomes too small, to make handling of each lens difficult in a process of incorporating each lens and to increase man-hour for each processing.
Namely, it is not preferable, from viewpoints of securing the working distance and of incorporating each lens, that a focal length of the two-group objective lens is made to be too small.
Further, in an optical pickup device, making a focal length of an objective lens to be small is disadvantageous for image height characteristics. The reason for the foregoing is that when trying to obtain an image height identical to that for the objective lens whose focal length is relatively great, an incident angle to an objective lens whose focal length is relatively small becomes great. The greater the incident angle is, the more astigmatism and coma deteriorate. Therefore, from the viewpoint of image height characteristics, it is not preferable that a focal length of an objective lens is too small.